In radio communications, signals are transmitted at a particular frequency, in a frequency band or in a number of frequency bands. The signals may be modulated in a variety of fashions using techniques such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and a multitude of other techniques. Nevertheless there are a finite number of available individual communications channels for separate sets of parties to communicate with each other. For example in TDMA 10 there are ten time slots for data to be encoded as separate channels on a bearer of a frequency band. In many radio communications systems such as GSM digital mobile radio protocol, the communications channel hops from one frequency band to another according to a specified routine. The system overcomes the effects of fading, scattering and other transmission problems on a particular channel by swapping channels and providing an average of the signal strength of the channels available, which will provide a sufficient signal Obstacles in a signal path, such as buildings in built-up areas and hills in rural areas, act as signal scatterers and can cause signalling problems. These scattered signals interact and their resultant signal at a receiving antenna is subject to deep and rapid fading and the signal envelope often follows a Rayleigh distribution over short distances, especially in heavily cluttered regions. A receiver moving through this spatially varying field experiences a fading rate which is proportional to its speed and the frequency of the transmission. Since the various components arrive from different directions, there is also a Doppler spread in the received spectrum. If the channel allocation was static, then as the subscriber, for example, moved to an urban environment where signal reflections affected the particular frequency in which the channel was operating more than other frequencies, then the channel which was previously best then becomes poor. In fact such movement may produce a break in communications. In fixed radio applications, the problems of fading still exist but are not so rapid; in a fixed system, the best channel would be likely to stay the best signal for a period of time.
In a cellular radio communication network, assuming uniform random distribution, there will be a tendency for the number of out stations to increase towards the edge of the range: three quarters of the area of a cell is occupied by the area defined by a sweep of the second half of the radius from the centre of an arc drawn from the centre of a base station. Problems occur, for example, due to increases in demand from regions on the edge of a cell where the vagaries of increased distance of an out station relative to a base station become more apparent. As the distance of an out station from a base station increases, then the contention for available channels increases.
One situation that frequently arises is that one will tend to get subscribers close to the base station being able to see all the frequency bearers being transmitted by the base station equally well--there may be some fading on some frequency bearers, but when in close proximity to the base station, a loss of 10 or 20 dB is of little or no consequence: the channel is a good channel. The further out that an outstation lies to the base station, then there will be an increase in the chance and severity of fading which can make some channels unusable whilst the other channels remain satisfactory or do indeed remain perfectly good. The situation will exist that a number of bearers, each of which contain a number of communication channels--and they need not be TDMA slots nor even slotted; just another physical resource that can be used--that the number of users on a particular bearer may be exhausted before full capacity is reached for the system. For example there may be a large number of subscribers in a particular area and the best signal channel is allocated even though the subscribers are in close proximity to the base station. Any further subscribers who wish to communicate on the bearer which has been swamped by the large number of subscribers in close proximity to the base station will be denied access to that particular channel. As systems in crease in their subscriptions, then these problems will become more frequently apparent.